Capping machines for the application of prethreaded closures onto prethreaded containers have been known for some time. In order to ensure that a prethreaded closure is not applied too tightly, which could possibly result in damage, conventional screw capping machines are provided with a screw capping head often called a "headset" having a torque dependent clutch. The clutch limits the maximum torque which can be transmitted to the prethreaded closure. Clutches of various types have been used. These have included slipping clutches and mechanical torque limiting clutches and also magnetic clutches. One example of a magnetic clutch is shown in U.S. Pat. No. 5,490,369 to Ellis et al., which is assigned to the Aluminum Company of America and which in incorporated herein by reference. Other examples of clutches are disclosed in U.S. Pat. Nos. 4,364,218; 4,492,068; 4,674,264; and 5,197,258. While many prior types of clutches have been generally satisfactory, many have not adapted themselves to ready adjustability, thus resulting in substantial down time when changes in the amount of applied torque are required due to different closures being applied, application forces required, etc. The Ellis et al., U.S. Pat. No. 5,490,369 teaches a capping head with an adjustable magnetic clutch consisting of opposed rings of magnet. One of the rings is disposed in a piston ring assembly that is adjustable relative to the other magnetic ring in order to vary the torque limit of the clutch.